Turns out the removal of the if() for messagesSlice triggered a wipe of the cached data on load for the message list. Adding an `if (!this.usingCachedNode)` fixes it, and better, more precise check than before.

https://github.com/jrburke/gaia/commit/f14ed8cf08e45c7a6dd08c8a0b4e9d7f10c45d9a

With that change,no more flash of empty message list.